That makes sense. There is no object / class named “Circle” built-in to Processing – so it doesn’t know what an ArrayList is. As far as I know there is also no Circle built-in to opencv…?
https://docs.opencv.org/java/2.4.3.1/overview-tree.html
Perhaps your original source for that line of code also included a .java file Circle.java, that contains a class definition…?
class Circle {
// Circle is defined here
}